For reinforcement with high strength C fibres, the optimized preparation parameters of two glass ceramics in the system MgO-Al2O3-SiO2 (MAS) and BaO-MgO-Al2O3SiO2 (BMAS) were investigated. The optimized preparation is related mainly to the mechanical properties of composites having a high portion of crystalline matrix microstructure. As a first step, the temperature ranges were determined at which the glass matrix crystallizes to a high degree. The optimized mechanical properties of the composites were obtained by systematic variation of the temperature-pressure profile for the densification of the prepregs. The best results were obtained by densifying the composition of each composite in the glass temperature region, T-g, before the matrix started to crystallize. Optima mechanical properties exist also for composite fibre concentrations observed in the range of 31-33 vol%.